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Thermal Expansion Tanks

08-06-2009, 10:37 PM

First, I've been lurking here for quite a while and have enjoyed the flow of information that is here.

My question: Why won't thermal expansion tanks work on the hot water side of the water heater. I've always been told to put them on the cold supply side, and that's where I've always put them. In the closed systems of today, does it really make a difference? I've even called AO Smith, the answer I get is "because it won't work there", but the can't seem to give me a reason why. Does anyone here have an answer?

Comment

On the hot side, it defeats the purpose of the expansion tank because the tank was designed for cold water.

It's a great sell when you see one on the hot side. Just show the customer the instructions that specifically say on the cold side and you got a easy repair.

I got one that has it on the hot side, one plastic 1/4 e.q. strap into drywall and the other metal strap 3" below it and connected up to the return duct. The unit is a 3 year old GE and the customer is trying to get it warrantied for the parts. They refuse to send him the correct parts....

He's a new customer who has already spent big bucks with us, so I'm being patient about this water heater.

Comment

I'm not a pro, but I've been learning about Expansion Tanks as I'm getting one for my place, so for what it's worth ...

The previous answers have already covered the primary reason for installing on the cold side, which is to prolong the life of the bladder. There are a couple other secondary considerations.

If the Expansion Tank is on the hot side, then when thermal expansion occurs heated water will flow into the Expansion Tank from the Water Heater. This water will start to cool as it sits in the Expansion Tank. When a faucet is opened or some other demand occurs to cause the pressure to return to normal, the cooler water in the Expansion Tank will flow back into the hot supply causing some momentary loss of hot water at the fixture. Not a big deal, but an undesireable condition.

A final consideration is that you may not have a choice depending on your local code. The code may state the expansion tank must be installed per the manufacturers instructions. So if the tank mfg says the cold side, that's where it needs to go if the code is strictly enforced. The code in some areas is more specific and mandates that the Expansion Tank go on the cold side (e.g., Illinois).

Comment

They will work but the bladder is more likely to fail when exposed to higher temperatures.

Some mfg. have a degree rating higher than typical WH settings but also have in their instructions to only install them on the cold side.

In short, they will work but I've not encountered a situation myself where I could not get it connected to the cold-closed-system.

J.C.

JC is exactly right but to add to this, the reason why it works on the cold side is because water can't compress. As the water heats up, it expands into the cold side because the hot side is closed. In the old days, the water expanded back into the city main, no big deal. Now with backflow protection, the water has nowwhere to expand to. Hence the entire need for expansion tanks.

Comment

Ok..... very good replies all....thank you. Now, let me throw a couple of wrinkles at you.

One: We use a lot of Amtrol tanks which have a max working temp of 200 degrees. 140 degree hot water supply shouldn't bother these tanks at all.

Two: On a recirculating system, there is a check valve on the cold supply to the WH to isolate the hot water system from the rest of the building. This would make the entire system closed whith the excess water having no where to go except the expansion tank, no matter on which side of the WH it is connected?

AO Smith says they immediately void warranties on WH when the Expansion tank is installed on the hot side, but so far, I'm failing to see the reasoning behind that.

Comment

I tend to be rather curious, usually about oddball things. I just didn't know if there was actually a valid reason for doing tanks the way we've been doing them or not. I guess now I'll have to find something different to think about for a while.